This invention relates to suppression of undesirable radio frequency (RF) signals in apparatus such as television receiver circuitry. Specifically, apparatus according to the invention presents a high impedance to RF signals which may be coupled to jumper wires which traverse the boundary between RF shielded area.
Radio frequency (RF) energy may be coupled from one circuit to another via exposed component leads by unintended coupling via stray circuit capacitance radiation. Such unwanted radio frequency signals may cause interference with signal processing circuitry and produce undesired effects, e.g., in a television receiver in the picture and sound perceived by the viewer.
A solution to the problem is to isolate the offending RF noise signal generating circuit, or the sensitive circuit receiving the noise signals, within a metal RF shield. It is usually necessary, however, to couple both signals and power between the shielded circuit and circuits external to the RF shield enclosure. Necessary signals and power may be coupled to and from the circuit enclosed within the RF shield is by providing apertures through the shield, sometimes called "mouseholes", through which to bring power and signal jumper wires to be connected to the isolated circuit. The mouseholes are, however, a breach in the RF shield which may allow stray RF energy to escape via the exposed jumper wires. Feed-through capacitors may be used to couple signals and power to and from the shielded circuit while significantly reducing interference from stray RF energy. Basically, a feed-through capacitor comprises a center conductor surrounded by a dielectric which is itself encased in a cylindrical metal outer conductor. The feedthrough capacitor extends through a hole in the shield and the outer conductor is soldered or otherwise attached to the shield. Feed-through capacitors are themselves expensive and do not readily lend themselves to automated assembly techniques. Attempts have been made to suppress the unwanted RF signals by placing ferrite beads in the RF signal path on jumper wires proximate to the shield wall on both sides, as taught in U.S. Pat. No. 3,913,038 (Carter). Unfortunately, this method still leaves some jumper wire exposed in and near the mousehole and the immediately adjacent area which may couple unwanted RF signal energy to nearby circuits.